Dwarf nova and accretion disks

Dwarf novae are eruptive variable stars exhibiting semi-periodic outbursts that recur on time scales of weeks to months with outburst amplitudes of 2-5 mag. The light curves of dwarf novae are rich in variations in different time scales, ranging from quasi-periodic eruptions with weeks to months, to very strict periodicities such as binary orbital periods of a few to several hours and the so-called superhump periodicity. Some strange phenomena occur in some of dwarf novae, such as superoutbursts and superhumps in SU UMa sub-class. Outburst mechanisms of dwarf novae are discussed. Outbursts and related phenomena can basically be understood by the disk instability model in that intrinsic instabilities in accretion disks cause time-varying light output from the disk. Two different intrinsic instabilities axe known in accretion disks of dwarf novae, the thermal instability and the tidal instability. With these two intrinsic instabilities combined, a rich variety in outburst behaviors of dwarf novae can be explained.